Electronic device, method for manufacturing the same, and silicon substrate for electronic device

ABSTRACT

An electronic device is formed by epitaxially growing a Si substrate on a Si layer of an SOI substrate in which the Si layer is deposited on a front surface of a substrate with an insulating layer interposed therebetween; forming an element on a front-surface side of the Si substrate; and forming a back-surface element aligned with respect to the element, on a back-surface side of the Si substrate after the substrate is etched. A mark is formed by etching and removing the Si layer and the insulating layer in a predetermined position of the SOI substrate. The element is formed using a concave part as a reference position. The concave part appears on the front surface of the Si substrate epitaxially grown on the mark. The back-surface element is formed using the mark as a reference position. The mark appears after the substrate is etched.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of co-pending application Ser. No.12/032,390, filed on Feb. 15, 2008, which claims priority to ApplicationNo. 2007-37881 filed in Japan on Feb. 19, 2007, the entire contents ofwhich are hereby incorporated by reference into the present applicationand for which priority is claimed under 35 U.S.C. §120.

BACKGROUND OF THE INVENTION

1. Technical Field

The invention relates to a silicon (Si) substrate or the like formanufacturing an electronic device, and more particularly to anelectronic device that is manufactured by aligning elements formed on afront surface thereof and elements formed on a back surface thereof withhigh precision, a manufacturing method thereof, and a silicon substratefor the electronic device.

2. Description of the Related Art

When an electronic device is manufactured on a silicon substrate, thereis a case where elements are formed on both a front surface and a backsurface of the silicon substrate and the elements formed on the frontsurface are aligned with respect to the elements formed on the backsurface. The “front surface” and “back surface” mean “one face” and “theother face” of a substrate, respectively.

For example, in a back-illuminated solid-state imaging device, asdescribed in Japanese Patent No. 3722367 (corresponding to US2004/005729 A), a plurality of photodiodes arranged in two-dimensionalarray and a signal reading-out circuit for reading out image signalsdetected by the photodiodes are formed on a front-surface side of thesemiconductor substrate, while color filters and micro lenses are formedon a back-surface side of the semiconductor substrate.

In the back-illuminated solid-state imaging device, light is incidentfrom a photographic subject to the back surface of the semiconductorsubstrate, and is received by the photodiodes provided on thefront-surface side. Accordingly, when the color filter and the microlens provided for each photodiode are not aligned with respect to thephotodiode, color mixture or the like occurs.

For this reason, it is necessary to align the elements (color filers,micro lenses) provided on the back-surface side with the elements(photodiodes, signal reading circuit, and the like) provided on thefront-surface side.

In Japanese Patent No. 3722367, a back-surface mark is formed whileviewing from the back-surface side through the semiconductor substrate,a metal layer or a silicide layer that is provided on the front-surfaceside. In this case, red or near-infrared light for which Si has lowabsorption rate is used as reference light. However, when a Siabsorption layer is formed to have a sufficient thick (e.g., 10 μm)capable of absorbing most of visible light, signal intensity is hardlycaught in the red reference light.

The near-infrared light having a wavelength of about 1.2 μm or less isabsorbed by Si. Accordingly, the near-infrared light is not suitable fora thick Si-layer structure, and aligning with high precision cannot beachieved.

US 2006/0186560 A proposes a method for manufacturing a back-illuminatedimage sensor using a Si substrate in which a p layer havingconcentration gradient is grown by epitaxial growth on an SOI substratewith a thin seed layer deposited thereon. However, US 2006/0186560 Adoes not mention about forming a lattice-shaped light-shielding layer, acolor filter, a micro lens, and the like on a back-surface side withhigh positional precision.

In the back-illuminated solid-state imaging element, when thelight-shielding layer, the color filter, the micro lens, and the likeprovided on the back-surface side deviates from a structure of a pixelportion formed on the front-surface side of the silicon substrate, colormixture occurs. For this reason, it is important to reduce the deviationin a coordinate system between an alignment mark provided on thefront-surface side and an alignment mark provided on the back-surfaceside.

When a size of a pixel becomes smaller than a thickness of a Si layer ofa photodiode in order to arrange a large number of pixels in an imagingdevice, color mixture may further occur. Accordingly, a micro lens, alight-shielding element, and the like are required. As a width of thelight-shielding element is increased, the color-mixture margin betweentwo adjacent colors is also increased accordingly, but it means openingsare narrowed as the color-mixture margin is increased, causing loss ofsensitivity. It is noted that the color-mixture margin is the tendencyof the color not to be mixed. For a micro pixel image sensor whichemploys a back-illuminated structure due to the decrease in pixel sizeand the shortage of sensitivity, there is a demand that thelight-shielding element is formed as narrow as possible. Therefore, ithas been desired to make the deviation in structure between thefront-surface layer and the back-surface layer as narrow as possible.

The back-illuminated solid-state imaging element has been describedabove as an example. However, the above description is also applicableto the overall semiconductor device for forming elements aligned withhigh precision on the front and back surfaces.

SUMMARY OF THE INVENTION

The invention provides an electronic device in which elements formed onfront and back surfaces are aligned with high precision, a method formanufacturing the same, and a silicon substrate for the electronicdevice.

According to an aspect of the invention, a method for manufacturing anelectronic device includes: forming a back-surface mark by etching andremoving a silicon layer and an insulating layer in a predeterminedposition of an SOI substrate in which the silicon layer is deposited ona front surface of a semiconductor substrate with the insulating layerinterposed between the silicon layer and the semiconductor substrate;epitaxially growing a silicon substrate for the electronic device, onthe silicon layer of the SOI substrate; forming a front-surface elementon a front-surface side of the silicon substrate for the electronicdevice using a concave part as a reference position, wherein the concavepart appears, on the front surface of the silicon substrate for theelectronic device epitaxially grown on the back-surface mark; andforming a back-surface element, that is aligned with respect to thefront-surface element, on a back-surface side of the silicon substratefor the electronic device after etching the semiconductor substrate ofthe SOI substrate, using the back-surface mark as a reference position,wherein the back-surface mark appears after the etching of thesemiconductor substrate of the SOI substrate.

According to another aspect of the invention, an electronic deviceincludes a silicon substrate, a front-surface element and a back-surfaceelement. The silicon substrate for the electronic device is epitaxiallygrown on a silicon layer of an SOI substrate in which the silicon layeris deposited on a front surface of a semiconductor substrate with aninsulating layer interposed therebetween. The front-surface element isformed on a front-surface side of the silicon substrate for theelectronic device. The back-surface element is formed on a back-surfaceside of the silicon substrate for the electronic device after thesemiconductor substrate of the SOI substrate is etched, with beingaligned with respect to the front-surface element. A back-surface markis formed by etching and removing the silicon layer and the insulatinglayer in a predetermined position of the SOI substrate. Thefront-surface element is formed using a concave part as a referenceposition, the concave part appearing, on the front surface of thesilicon substrate for the electronic device epitaxially grown on theback-surface mark. The back-surface element is formed using theback-surface mark as a reference position. The back-surface mark appearsafter the semiconductor substrate is etched.

Also, the electronic device may be a back-illuminated solid-stateimaging element. The front-surface element may include a photoelectricconversion element and a signal reading-out unit. The back-surfaceelement may include an optical element opposed to the photoelectricconversion element.

Also, the signal reading-out unit may include a charge transfer unit.

According to further another aspect of the invention, a siliconsubstrate for electronic device is manufactured by: epitaxially growinga silicon substrate on a silicon layer of an SOT substrate in which thesilicon layer is deposited on a front surface of a semiconductorsubstrate with an insulating layer interposed therebetween; forming afront-surface element on a front-surface side of the silicon substrate;and forming a back-surface element, that is aligned with respect to thefront-surface element, on a back-surface side of the silicon substrateafter etching the semiconductor substrate of the SOI substrate. Aback-surface mark is formed by etching and removing the silicon layerand the insulating layer in a predetermined position of the SOIsubstrate. The front-surface element is formed using a concave part as areference position. The concave part appears, on the front surface ofthe silicon substrate for the electronic device formed on theback-surface mark. The back-surface element is formed using theback-surface mark as a reference. The back-surface mark appearing afterthe semiconductor substrate is etched.

According to the invention, a concave part formed on the basis of aback-surface mark is used as a front-surface mark, front-surfaceelements are positioned with reference to the front-surface mark, thehack-surface elements are positioned with reference to the back-surfacemark, and thus it is possible to align the front and back-surfaceelements so as to be opposed to each other with high precision.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional schematic diagram illustrating a back-illuminatedsolid-state imaging element according to an embodiment of the invention.

FIG. 2 is a diagram illustrating a manufacturing process of theback-illuminated solid-state imaging element shown in FIG. 1.

FIG. 3 is a diagram illustrating a manufacturing process after themanufacturing process shown in FIG. 2.

FIG. 4 is a diagram illustrating a manufacturing process after themanufacturing process shown in FIG. 3.

FIG. 5 is a diagram illustrating a manufacturing process after themanufacturing process shown in FIG. 4.

FIG. 6 is a diagram illustrating a manufacturing process after themanufacturing process shown in FIG. 5.

FIG. 7 is a diagram illustrating a relation between a front-surface markand a back-surface mark shown in FIG. 6.

FIG. 8 is a sectional schematic diagram illustrating an electronicdevice according to another embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Hereinafter, embodiments of the invention will be described withreference to the drawings.

FIG. 1 is a sectional schematic view of a back-illuminated solid-stateimaging element 100 according to an embodiment of the invention. Thesolid-state imaging element according to the embodiment is an interlinetype CCD. Vertical charge transfer paths (VCCD) 21 and photodiodes(photoelectric conversion elements) 22 are formed on a front-surfaceside of a p type semiconductor substrate 1. A color filter (red (R),green (G) and blue (B)) layer 23 and micro lenses 24 are laminated on aback-surface side thereof.

The back-illuminated solid-state imaging element 100 according to theembodiment is a CCD type. However, the invention is applicable to a CMOStype solid-state imaging element or other solid-state imaging elementsin the same manner as described in Japanese Patent No. 3722367.

A high-concentration p layer 25 is formed on a surface of thesemiconductor substrate 1 on the back-surface side, and the p layer 25is grounded. An insulating layer 26, such as silicon oxide or siliconnitride, which is transparent with respect to incident light, isdeposited on the high-concentration p layer 25. A high refractive indexlayer 27, which is transparent with respect to the incident light, suchas silicon nitride or a diamond-structured carbon film, is depositedthereon. The color filter layer 23 and the micro lens (top lens) layer24 are sequentially deposited thereon. Each micro lens 24 is formedbased on an alignment mark which will be described later so as to focuson the center of the corresponding photodiode 22 opposed to each lens24.

The color filter layer 23 is arranged in pixel (photodiode) units, and alight-shielding element 28 for preventing color mixture between pixelsis provided in the color filter layer 23 on one side of the color filterlayer 23 close to the semiconductor substrate 1 and between the adjacentpixel units.

The vertical charge transfer path (VCCD) 21 formed on the front-surfaceside of the semiconductor substrate 1 includes a buried channel 31 of ahigh-concentration n layer and a transfer electrode film 33 which aredeposited with a gate insulating film 32 interposed therebetween. Thegate insulating layer 32 is formed of a silicon oxide film or aninsulating film having an ONO (silicon-oxide film, silicon-nitride film,and silicon-oxide film) structure formed on the outermost surface on thefront-surface side of the semiconductor substrate 1.

The vertical charge transfer paths 21 are formed to extend in adirection perpendicular to a direction in which a horizontal chargetransfer path (HCCD) (not shown) extends. A number of vertical chargetransfer paths 21 are formed. Between the adjacent vertical chargetransfer paths 21, a plurality of photodiodes 22 are formed at apredetermined pitch in a direction along the vertical charge transferpaths 21.

In the embodiment, each photodiode 22 includes an n layer 35 formed onthe front-surface side of the p type semiconductor substrate 1 and an n⁻layer 36 formed below the n layer 35. A thin and high concentration ptype layer 38 for suppressing dark current is formed on a front-surfaceportion of the n layer 35. An n⁺ layer 39 as a contact portion is formedon a central surface portion of the surface layer 38.

A p layer 41 having a higher p concentration than that of the substrate1 is formed below the buried channel (n⁺ layer) 31 of the verticalcharge transfer path 21. A p⁺ region 42 serving as an element separatoris formed between (i) the n layer 31 and p layer 41 and (ii) thephotodiode 22 located right from then layer 31 and p layer 41, as shownin FIG. 1. A p⁺ region 42′ having higher concentration than that of thesemiconductor layer 1 is formed below each p layer 41, and each p⁺region 42′ separates two adjacent photodiodes 22. Each p⁺ region 42′ isprovided parting a boundary portion between adjacent pixel units, thatis, a portion corresponding to the light-shielding member 28.

The p layer 41 formed below the buried channel 31 of the vertical chargetransfer path 21 extends to a surface end portion of the n layer 35,which is located on the left side of the p layer 41, in the shownexample. The p+ surface layer 38 at the end portion is located at aportion retreating from a right end surface of the n layer 35. The leftend portion of the transfer electrode film 33 extends to overlap withthe p layer 41 up to the left end portion of the p layer 41. The surfaceend portions of the transfer electrode film 33 and the p layer 41slightly overlap with the n layer 35.

Such an overlapping configuration can be achieved because there is asufficient area on the front-surface side of the semiconductor substrate1 in the back-illuminated type. There is no sufficient area in thefront-surface illumination type where light from a photograph subject isincident onto a front surface provided with the photodiode and thesignal reading circuit. Accordingly, the end portion of the transferelectrode film can be extended only up to a position corresponding tothe end portion of the photodiode. Thus, it is difficult to interpose ap layer therebetween.

As in this embodiment, when the p layer 41 is interposed between thetransfer electrode film 33 and the n layer 35, it is possible todecrease a reading-out voltage applied to the transfer electrode film(used also as a reading-out electrode) 33 and to reduce powerconsumption of the CCD solid-state imaging device.

On the insulating layer 32 formed on the outmost surface of thesemiconductor layer 1, the transfer electrode film 33 such as apolysilicon film is formed. The insulating layer 45 is depositedthereon. Openings are formed in the insulating layers 32 and 45 on then⁺ layer 39, and a metallic electrode 46 is deposited on the insulatinglayer 45, thereby connecting the electrode 46 to the n⁺ layer 39. Theelectrode 46 serves as an overflow drain of the back-illuminatedsolid-state imaging device 100.

When an image of a photographic subject is imaged by theback-illuminated solid-state imaging device 100 having such aconfiguration, light from the photographic subject is incident throughthe back surface of the semiconductor substrate 1. The incident light isconcentrated by the micro lens 24, passes through the color filter layer23, and then enters the semiconductor layer 1.

When the light concentrated by the micro lens 24 enters thesemiconductor substrate 1, the light travels while being concentratedtoward the photodiode 22 corresponding to the micro lens 24 and thecolor filter 23. The light is gradually absorbed in the semiconductorsubstrate 1, and the light is photoelectrically converted to generatepairs of holes and electrons.

In the back-illuminated solid-state imaging device 100, a distance fromthe back surface of the semiconductor 1 to the n region 22 of thephotodiode is about 9 μm. Accordingly, all the incident light isgradually absorbed in the semiconductor substrate 1 andphotoelectrically converted until the light reaches the n⁺ regionprovided on the front-surface side of the semiconductor substrate 1,that is, the charge transfer path 21. Therefore, it is unnecessary toshield the vertical charge transfer path 21 from light.

The electrons generated in the photoelectric conversion region (a regionfrom the p layer 25 to the n region 35) of each pixel are accumulated inthe n region 35 of the corresponding pixel. When the reading-out voltageis applied to the transfer electrode film 33, which is also used as thereading-out electrode, the electrons are read out from the n region 35to the buried channel 31 on the right side in the shown example. Then,the electrons are transferred to the horizontal charge transfer path(not shown) along the vertical charge transfer path 21, and aretransferred to an amplifier along the horizontal charge transfer path.The amplifier outputs a signal voltage based on the amount of signalcharges as an image signal.

Next, a manufacturing process of the back-illuminated solid-stateimaging device 100 will be described with reference to FIGS. 2 to 7.First, an SOI (Si-on-insulator) substrate 50 shown in FIG. 2 isprepared. The SOI substrate 50 includes a thick silicon substrate 51, anoxide film layer 52 formed on a surface of the thick silicon substrate51, and a thin silicon layer 53 formed on a surface of the oxide filmlayer 52.

Then, as shown in FIG. 3, marks 54 are formed in predetermined positionsof the surface of the SOI substrate 50. The marks 54 are formed byetching the thin silicon layer 53 and the oxide film layer 52 therebelowto expose the surface of the silicon substrate 51.

Then, as shown in FIG. 4, a p type silicon layer 1 having a thickness ofabout 10 μm is epitaxially grown on the surface of the SOI substrate 50provided with the marks 54. The p type silicon layer 1 serves as the ptype silicon substrate 1 in FIG. 1.

When the p type silicon layer 1 is epitaxially grown on the Si layer 53of the SOI substrate 50 and on the marks 54 that exposes the siliconsubstrate 51, concave parts 55 are formed on a surface of the p typesilicon layer 1 while being aligned with the mark 54.

As shown in FIG. 5, using the concave parts 55 serving as alignmentmarks, front-surface elements such as the buried channel 31 and thetransfer electrode film 33 formed thereon described with reference toFIG. 1, the photodiodes (not shown in FIG. 5), and an aluminum wiringlayer 56 are formed on the front-surface side of the p type siliconlayer 1.

After the elements are formed on the front-surface side of the p typesilicon substrate 1, a supporting substrate 57 that supports the p typesilicon substrate 1 is laminated onto the front surface.

Then, as shown in FIG. 6, the silicon substrate (original substrate) 51of the SOI substrate 50 shown in FIG. 5 is removed by etching. Forexample, when chemical etching is performed using KOH, the siliconsubstrate 51 is etched and the etching stops at the oxide film layer 52.However, the etching slightly progresses in positions of the marks 54because there is no oxide film layer 52. The marks 54 are exposed asconcave parts on the back-surface side of the p type semiconductor layer1.

Then, although not shown, optical elements such as the color filters 23,the light-shielding members 28, and the micro lenses 24 described inFIG. 1 are formed on the back-surface side of the p type siliconsubstrate 1, using the alignment marks 54 as the aligning basis.

Accordingly, it is possible to align the back-surface elements with thefront-surface elements with high precision, and it is also possible tomake the width of the light-shielding element 28 as narrow as possible,thus preventing color mixture.

In FIGS. 2 to 6, the marks 55 which are formed above the mark 54 duringthe growth of the p type epitaxial layer 1 have the same size as themarks 54. However, actually, the size of the marks 55 gets narrower andshallower as the epitaxial layer 1 gets thicker as shown in FIG. 7.Accordingly, it is preferable to form the marks 54, taking the decreasein size into consideration. And, it is also preferable to form othermarks based on the mark 55 instead of using the marks 55 directly asaligning basis, and then to form elements on the front-side surfacewhile the formed elements are aligned with the other marks as thealigning basis.

In the embodiment described above, the back-illuminated solid-stateimaging device 100 is described as an example. However, the embodimentdescribed above is also applicable to general semiconductor devices inwhich elements and patterns are formed on both front and back surfaces.

For example, in an electronic device shown in FIG. 8, a semiconductorintegrated circuit 59 is formed on the front surface thereof, a Sisubstrate is used as the supporting substrate 57, and the Si substrate57 is attached to the front surface of a substrate 101 for an electronicdevice with the Van der Waals' force. When the silicon substrate 51 ofthe SOI substrate 50 is etched, the silicon substrate 51 below the oxidefilm 52 is left thinly as a Si layer and devices such as an FET 58 areformed on the front surface of the thinly-left Si layer 51.

When the electronic devices 58 are formed on the back-surface side ofthe substrate 101, the marks 54 are not exposed to outside as “concaveparts.” However, the marks 54 are easily visible because the siliconlayer 51 is thin. In addition, the marks 54 are easily visible becausethe crystallinity at the marks 54 is different from the vicinity of themark 54 in the early stage of the epitaxial growth.

A silicon layer may be formed on the oxide film layer 52 after whollyremoving the silicon substrate 51 by etching and then, an element may beformed on the silicon layer rather than on the semiconductor substrate51. For example, the electronic devices may be formed on the oxide filmlayer 52 by the use of a polysilicon layer or an amorphous silicon layerformed at low temperature, or a silicon crystal layer formed byannealing them using a laser.

As described above, according to the embodiment, when the elements areformed on both front and back surfaces of the silicon substrate forelectronic device 1, it is possible to align the front-surface elementsand the back-surface elements with high precision and to manufacture thehigh-performance and high-quality electronic devices with low cost.

In the silicon substrate for electronic device according to theinvention, it is possible to manufacture the high-performance andhigh-quality electronic devices because it is possible to perform highlyprecise alignment between the front surface and the back surface. Inaddition, when the back-illuminated solid-state imaging element ismanufactured, it is possible to manufacture a solid-state imagingelement with high aperture ratio, preventing color mixture.

1. An electronic device comprising: a silicon substrate for theelectronic device that is epitaxially grown on a silicon layer of an SOIsubstrate in which the silicon layer is deposited on a front surface ofa semiconductor substrate with an insulating layer interposedtherebetween; a front-surface element formed on a front-surface side ofthe silicon substrate for the electronic device; and a back-surfaceelement formed on a back-surface side of the silicon substrate for theelectronic device after the semiconductor substrate of the SOI substrateis etched, with being aligned with respect to the front-surface element,wherein: a back-surface mark is formed by etching and removing thesilicon layer and the insulating layer in a predetermined position ofthe SOI substrate, the front-surface element is formed using a concavepart as a reference position, the concave part appearing, on the frontsurface of the silicon substrate for the electronic device epitaxiallygrown on the back-surface mark, and the back-surface element is formedusing the back-surface mark as a reference position, the back-surfacemark appearing after the semiconductor substrate is etched.
 2. Theelectronic device according to claim 1, wherein the electronic device isa back-illuminated solid-state imaging element, the front-surfaceelement comprises a photoelectric conversion element and a signalreading-out unit, and the back-surface element comprises an opticalelement opposed to the photoelectric conversion element.
 3. Theelectronic device according to claim 2, wherein the signal reading-outunit comprises a charge transfer unit.
 4. A silicon substrate forelectronic device manufactured by epitaxially growing a siliconsubstrate on a silicon layer of an SOI substrate in which the siliconlayer is deposited on a front surface of a semiconductor substrate withan insulating layer interposed therebetween; forming a front-surfaceelement on a front-surface side of the silicon substrate; and forming aback-surface element, that is aligned with respect to the front-surfaceelement, on a back-surface side of the silicon substrate after etchingthe semiconductor substrate of the SOI substrate, wherein a back-surfacemark is formed by etching and removing the silicon layer and theinsulating layer in a predetermined position of the SOI substrate, thefront-surface element is formed using a concave part as a referenceposition, the concave part appearing, on the front surface of thesilicon substrate for the electronic device formed on the back-surfacemark, and the back-surface element is formed using the back-surface markas a reference, the back-surface mark appearing after the semiconductorsubstrate is etched.